Some brushless motors that eliminate a brush and commutator segment from a DC (direct current) motor and have an electronic rectification mechanism have a rotor with a bipolar permanent magnet and U-phase, V-phase and W-phase windings for generating a magnetic field around the rotor to rotate the rotor.
Furthermore, in some brushless motors, there is a sensorless brushless motor that is not provided with a magnetic pole sensor for detecting the position of a magnetic pole of a rotor.
The sensorless brushless motor applies a motor drive current to two of three motor windings to rotate the rotor. Consequently, induced electromotive force is generated in one remaining winding as a result of the rotation of the motor. Then, the position of a magnetic pole of the rotor is detected based on the value of the generated induced electromotive force, and switching control is carried out on currents (drive voltage vectors) of the motor windings based on the position of the magnetic pole.
There is a PLL (Phase Locked Loop) control method as one of the control methods for stabilizing the rotational speed of such a brushless motor.
The PLL control method is a method of matching the phase of a frequency corresponding to the reference rotational speed with the phase of a frequency to be compared.
The PLL control method providing a high degree of stability in the rotational speed is often used in a motor that requires highly precise speed control, such as, for example, a motor incorporated in a turbo-molecular pump or other vacuum pump or in a spindle.
The speed control performed on a motor by the PLL control method requires at least approximately 20 Hz (hertz) of input signal frequency for comparison.
In other words, for example, in a control circuit where 1 pulse of input signal is generated per rotation of a rotor, a PLL circuit cannot be locked (operated) if the rotor does not rotate approximately 20 times per second.
Therefore, energization control is performed on the winding of the motor by an open-loop until the rotational speed of the rotor increases to a certain rotational speed by which the PLL circuit can be locked. Specifically, when starting up the motor, switching control of the drive voltage vector to the motor is executed without feedback control.
For example, the drive voltage vector is first switched at a low frequency (low speed) close to a DC (direct current), and the rotor is caused to follow the switching speed as it is gradually increased. In this manner, the rotational speed of the rotor is increased to the rotational speed by which the PLL circuit can be locked.
As described above, the time period (start-up time period) during which the rotational speed of the rotor is increased to the rotational speed for locking the PLL circuit is subjected to drive control by an open-loop. Therefore, when the speed of switching the drive voltage vector increases too fast or the load on the rotor changes rapidly, the rotor cannot follow the magnetic field formed by the winding of the motor and loses its synchronism, failing in starting up.
Furthermore, in the case where the rotor needs to be restarted due to a blackout or the like during the start-up time period, the position of a magnetic pole cannot be detected in a sensorless brushless motor. Thus, the rotor has to be restarted after braking and stopping the rotor once with a direct current.
Technology for achieving stability in control of the start-up time period of the motor and thereby reducing the start-up time period of the motor has conventionally been proposed by Japanese Patent Application Laid-open No. 2002-176793.
Japanese Patent Application Laid-open No. 2002-176793 discloses a technology for detecting the position of a magnetic pole of a rotor based on induced electromotive force generated in the U-phase winding of the motor, which is not energized, and starting up the motor by repeatedly performing the energization from the V phase to the W phase and the energization from the W phase to the V phase sequentially in response to the detected magnetic pole position. Hereafter this start-up method is taken as a first start-up method.
Japanese Patent Application Laid-open No. 2002-176793 also discloses a technology for detecting the position of the magnetic pole of the rotor based on the potential difference between the U phase and the V phase, and starting up the motor by repeatedly performing the energization from the V phase to the W phase and the energization from the W phase to the U phase sequentially in response to the detected magnetic pole position. Hereafter this start-up method is taken as a second start-up method.